Non-aqueous dispersions are becoming more and more important. They are used in particular as electro-rheologic fluids (ERF). Electro-rheologic fluids are understood to be dispersions of finely divided particles in hydrophobic and electrically non-conducting oils. The apparent viscosity of these dispersions changes very rapidly and reversibly from the liquid to the plastic or solid state under the effects of an electric field. The apparent viscosity is altered by both constant electric fields and alternating electric fields, wherein the flow of current through the ERF should be very small.
The increase in viscosity in an ERF on application of an electrical field can be explained qualitatively as follows: the colloidal, chemically stable, dispersed particles polarise in the electric field and agglomerate due to dipole interaction in the direction of the field lines. This leads to an increase in the apparent viscosity. The agglomeration procedure is reversible, i.e. if the electric field is switched off, the particles redisperse and the viscosity returns to the original value.
The electrical polarisability of the disperse phase is, therefore, an important prerequisite to the production of the electro-rheological effect. Therefore ionic or electronically conductive materials are often used as the disperse phase.
In some of the ERFs from the prior art, the disperse phase comprises organic solids such as, for example, ion-exchange resins (U.S. Pat. No. 3,047,507) or silicone resins (U.S. Pat. No. 5,164,105). In some cases, however, coated inorganic materials such as zeolites (U.S. Pat. No. 4,744,914) or silica gel (U.S. Pat. No. 4,668,417) are used. In the case of the substances mentioned, the electro-rheological effect is attributed to the solids being loaded with water. Small amounts of water increase the ionic conductivity and thus the polarizability of the disperse particles which is essential for producing the effect. Water-containing systems, however, are not very stable. Solids such as metal powders or zeolites have the disadvantage that they are abrasive. The abrasive effect can be greatly influenced by the choice of disperse phase. Therefore polymeric substances, in particular elastomers, are preferred to inorganic powders as disperse phases.
ERFs can be used wherever forces need to be transferred with the assistance of a low electrical power such as, for instance, in couplings, hydraulic valves, shock absorbers, vibrators or devices for positioning and fixing workpieces.
In addition to the general requirements for ERFs such as a good electro-rheological effect, high thermal stability, low power consumption and resistance to chemicals, the abrasiveness, basic viscosity and sedimentation stability of the disperse phase play an important part in the practical application of ERFs. The disperse phase should form as little sediment as possible, and in any case should be readily redispersible and also should not cause any abrasion under extreme mechanical stress.
An effective electro-rheological fluid should, therefore, have a low basic viscosity and a high viscosity after application of the electric field, i.e a large change in viscosity. As is well-known, the ER effect increases with the proportion by volume of disperse phase. Achieving a low basic viscosity with a high proportion of solids depends firstly on the shape and on the particle size distribution of the disperse phase and secondly on the dispersing effect of any dispersing aids used. In addition, the conductivity of the disperse phase depends on the particle size. Optimising these parameters is only possible by precise adjustment of the particle sizes or of the particle size distribution in the disperse phase. Traditional processes for preparing non-aqueous dispersions are based on milling solid substances to the desired particle size, followed by dispersion in the dispersion medium. U.S. Pat. No. 5,268,118 disclosed the spray-drying of polymer solutions or reactive monomer mixtures followed by dispersion of the solidified particles. In both processes, regulation of the particle sizes and their distribution is only possible by means of a costly classification and separation procedure for the particles produced. U.S. Pat. No. 4,996,004 describes non-aqueous dispersions of polyether polymers which were prepared by milling and subsequent dispersion in non-aqueous liquids. In U.S. Pat. No. 5,073,282, hydrophobic polymer dispersions were prepared by precipitation polymerization in a non-polar carrier liquid and loaded with a second, polar liquid in order to produce electro-rheologic fluids. Here, the particle sizes and their distribution are determined by the polarity of the monomers and solvents and can be varied only within narrow limits. There is, therefore, a need for a process for preparing non-aqueous dispersions which contain a high proportion of solids, which are sedimentation stable and which have a low basic viscosity and a large change in viscosity following the application of an electric field.
The object of the present invention was, therefore, to provide a simple process which enables the preparation of non-aqueous dispersions with this range of properties.